A lunar core dynamo limited to the Moon’s first ~140 million years

Single crystal paleointensity (SCP) reveals that the Moon lacked a long-lived core dynamo, though mysteries remain. An episodic dynamo, seemingly recorded by some Apollo basalts, is temporally and energetically problematic. We evaluate this enigma through study of ~3.7 billion-year-old (Ga) Apollo basalts 70035 and 75035. Whole rock analyses show unrealistically high nominal magnetizations, whereas SCP indicate null fields, illustrating that the former do not record an episodic dynamo. However, deep crustal magnetic anomalies might record an early lunar dynamo. SCP studies of 3.97 Ga Apollo breccia 61016 and 4.36 Ga ferroan anorthosite 60025 also yield null values, constraining any core dynamo to the Moon’s first 140 million years. These findings suggest that traces of Earth’s Hadean atmosphere, transferred to the Moon lacking a magnetosphere, could be trapped in the buried lunar regolith, presenting an exceptional target for future exploration.


Mineral Alteration in Lunar Samples
The formation of lunar magnetic minerals form from the reduction of precursor phases, and as such this is fundamentally different from the formation processes of common magnetic phases in typical terrestrial rocks 1−2 .The generally reduced phases are susceptible to alteration in the laboratory with applied heatings needed for robust paleointensity analyses, involving chemical and/or structural changes 1,3 .A controlled (inert) atmosphere is sometimes used in thermal demagnetization experiments on terrestrial rocks to limit alteration in the laboratory.In some cases this is successful, but in others it leads to reduction and the formation of new magnetic minerals, compromising magnetic data (see discussion in ref 4).The use of a highly reducing atmosphere 5 , mimicking that thought to exist on the Moon during the formation of lunar magnetic minerals, is problematic because it could lead to the further reduction of Fe-bearing phases, producing new magnetic minerals, and rendering any magnetic data bearing on the ancient Moon unusable.Moreover, this approach does not address structual changes 3 .
An alternative approach is to concentrate on kinetics, and rapid heating/cooling to limit mineral alteration.This is achieved in this and prior studies using CO 2 lasers 6−7 .Ultimately, the effectiveness of this approach must be tested in each experiment, and this is done by means of TRM tests.In this study, there are several levels of tests.First, after the initial heating in an applied field, the sample is reheated to the same temperature in a zero field.If new magnetic minerals had formed, the magnetization should not return to the zero state as magnetic interactions amongst the newly formed phases would tend to leave a residual magnetization.Second, after the heating in an initial field (20 µT in this case) our crystals are reheated to a field twice as strong (40 µT).Magnetic mineral formation is time dependent, and if new magnetic minerals are being formed by the applied laboratory heatings we would expect that the magnetization after the second heating should be much larger than expected, corresponding to a magnetization efficiency >>100%.These values are not observed in the crystals studied here, or in prior work 1 .Third, the effects of alteration are variable depending on initial Fe-beating phase content (chemistry and amount), so we require replication for all studies.The SCP results reported here and in prior studies 1 have replicates.Therefore, claims in ref 5 that the lunar SCP values might reflect alteration are erroneous because tests and data specifically exclude alteration.

Debates on Impact Magnetization in Lunar Samples
Impacts can create large magnetic fields through charge separation, a well-established process documented in laboratory experiments 8 and studied through modeling of asteroid 9 and cometary impacts 10 .There are now three Apollo samples that provide evidence for magnetization from an impact plasma: Apollo 70019, 15498, and 64455.Studies of 70019 11 impact glass lacked pTRM checks, but the regular linear decay of remanence with increasing demagnetization alone is strong evidence for an impact magnetization.Magnetizations from Apollo breccia 15498 were originally reported by Gose et al. 12 who interpreted these as of internal origin.Glass from the sample was studied by Tikoo et al. 13 who also called for a core dynamo, but this interpretation is contradicted by the new paleointensity data as well as unmagnetized parts of the sample.Tarduno et al. 1 highlighted that the 15498 glass data that yield high unblocking temperature magnetizations record essentially null fields at intermediate to low temperatures (250 to 540 o C) and are more compatible with the presence and then absence of a magnetizing field, as might happen if the 15498 passed through an impact plasma, or that plasma dissapated.This interpretation is also consistent with the unmagnetized glass of 15498.Arguably the best impact magnetization recorder is Apollo 64455 because this ballistic-shaped glass-coated sample 14−15 most likely traveled through the impact plasma that created South Ray crater.Thermal and nonthermal techniques yield paleointensities that match independent predictions from impact modeling 1 .
Proponents of a lunar dynamo have attempted to criticize the 64455 data in several ways.We note that impact magnetization of 64455 is not required for our explanation of our WRP values because this is supported by the independent work on impact plasma magnetization 8−10 .Nevertheless we address the critiques here for completeness, and to further illustrate how the complexities of Apollo magnetic data can lead to erroneous conclusions.The following comments were made in several coordinated abstracts presented at the 2023 LPSC conference.One claim is that the signal might be due to alteration 5 , another that the signal could be due to terrestrial contamination in other laboratories 5,16 , another is that there no impact magnetization based on studies of other Apollo glasses and other samples from 64455 16−17 , and finally it was claimed that cooling rates in glass are too long to record impact fields 5 .We explain below why the 64455 data contradict each claim.
Alteration can occur with short-duration heating, and that is why pTRM checks are essential.These checks exclude alteration in the 64455 specimens that pass reliability criteria 1 .Twelve percent of the 64455 glass specimens examined in Tarduno et al. 1 passed.This value is typical of terrestrial results 18 but exceedingly high for lunar samples.The experimental alteration checks show that the suggestion of alteration (ref 5) is incorrect.
The 64455 sample studied in Tarduno et al. 1 was a return sample meaning that it had been previously supplied to a different scientist.However, the splits (specimens) used from the sample used by Tarduno et al. 1 can not be the same as in prior studies.That is Leich et al. 19 noted that they studied glass separated from the ground mass ("chipping the sample from the rock" 19 ), whereas the glass studied in Tarduno et al. 1 was attached, and subsequently subsampled into 1 mm-sized pieces used for analysis.Moreover, a typical lab contamination signal consists of a single coherent vector (overprint) affecting the entire sample.Tarduno et al. 1 described different behavior in specimens.Specifically "...evidence for multiple components and/or changing directions after field-off thermal treatments" was seen in samples rejected for paleointensity determination.This differential behav-ior, reported in more detail in Cottrell et al. 20 , represents an internal test that reveals the claim of terrestrial contamination (claim of refs 5 and 16) to be spurious.
Apollo 64455 is different from other "splash" glasses of the Apollo 17 collection, and therefore it would not be surprising if some of these cooled after the impact plasma associated with South Ray crater dissapated.However, the data of ref 16-17 are obtained with alternating fields are highly scattered and appear to be dominated by GRM magnetizations, exacerbated by a very large number of AF steps.Because of this methodological limitation, we conclude that these data are of insufficient number, quality and technique (e.g., high quality thermal data are needed) to support conclusions on the presence/absence of lunar impact fields.Moreover, we note that the 64455 magnetization reported in Tarduno et al. 1 has recently been reproduced by independent measurements 21 .
Tarduno et al. 1 explained that simple conductive cooling models likely do not yield accurate measures of glass quenching time, and See et al. 22 emphasized that condensation nucleate within the glass could greatly affect prior calculations and more generally that cooling could have been on second timescales.We further note that the great variety of FeNi textures preserved in 64455 glass 1 demand rapid quenching, but it is certainly possible, if not likely, that some glass specimens might have magnetized particles and others might not, because of a hetergenous distribution of the inclusions and the relatively rapid decay of the impact plasma magnetization.Finally, we note that the lifetime of the magnetizing field quoted in ref 5 is erroneous because it does not account for the decay of the field (i.e., an impact plasma field will be present for much longer than the peak field values available from refs.8-10).